Healthy adults have a closely interrelated nutritional, metabolic, and immune system that regulates food requirements and responds well to short-term nutritional deprivation but not to nutrient excess.1 However, when critically ill, patients no longer have control over their food intake, and clinicians may administer nutritional support with little understanding of individual patient needs in the light of inflammatory demands, injury responses, and the underlying influences of genotype and age.2 Because robust evidence about the nutritional requirements for critically ill patients at the various stages of their illness is lacking, particularly when they are acutely ill,3 clinicians often base nutritional support on “requirements” based on averages and guesswork.